Method of manufacture improving the behaviour of welded heat-resistant steel parts

ABSTRACT

Method of manufacture improving the behaviour of welded pieces of heat-resistant steel withstanding working temperatures ranging from 900* to 1,050*C. According to the invention the pieces are made of high-alloy austenitic steels alloyed with chromium, nickel and/or cobalt and containing less than 60 percent iron and up to 0.5 percent carbon with small concentrations of additional elements such as manganese and silicon and the weldings are subjected to a heattreatment at a temperature comprised between about 1,100*C and 1,200*C during a period of time of from several minutes to a few hours.

United States Patent [191 Bellot et al.

[451 Feb. 11, 1975 METHOD OF MANUFACTURE IMPROVING THE BEHAVIOUR OF WELDED HEAT-RESISTANT STEEL PARTS [75] Inventors: Jean Bellot, Pompey; Michel Hugo,

Custines;' Jacques Thuillier, Pont de LArche; Roger Hubert, Rouen, all arr r565 WM [73] Assignee: Societe Des Acieries Du Manoir-Pompey, Neuilly S/Seine, France [22] Filed: Feb. 8, 1973 [21] Appl. No.: 330,635

[30] Foreign Application Priority Data Dec. 15, 1972 France 72.44814 [52] US. Cl 148/127, 29/196.l, 148/34 [51] Int. Cl B32b 15/18, C2ld 9/50 [58] Field of Search..... 148/34, 127; 29/196, 196.], 29/487, 497, 498

[56] References Cited UNITED STATES PATENTS 2,200,229 5/1940 Strauss 29/1961 2,544,336 3/1951 Linnert 29/1961 OTHER PUBLICATIONS Welding Handbook, 1942 Ed., pp. 801 804.

Primary ExaminerC. Lovell Attorney, Agent, or FirmKenyon-& Kenyon Reilly Carr & Chapin [57] ABSTRACT Method of manufacture improving the behaviour of welded piecesof heat-resistant steel withstanding working temperatures ranging from 900 to 1,050C.

According to the invention the pieces are made of high-alloy austenitic steels alloyed with chromium, nickel and/or cobalt arid containing less than 60 percent iron and up to 0.5 percent carbon with small concentrations of additional elements such as manganese and silicon and the weldings are subjected to a heat-treatment ata temperature comprised between about 1,100C and 1,200C during a period of time of from several minutes to a few hours.

8 Claims, 5 Drawing Figures PATENTED FEB] I I975 SHEET 1 OF 3 PATENTEDFEBI 1 I975 SHEET 2 [1F 3 METHOD OF MANUFACTURE IMPROVING THE BEHAVIOUR OF WELDED HEAT-RESISTANT STEEL PARTS The present invention has essentially for its object a method of manufacture improving the behaviour of welded parts or pieces of heat-resistant steel withstanding working temperatures on the order of from 900C to 1,050C.

It is known that for high working temperatures ranging from about 900 to l,050C, use is made industrially of heat-resistant austenitic steels having excellent properties of resistance to corrosion, in particular to oxidation, as well as a very good creep resistance at such working temperatures.

Such heat-resistant steels are used more particularly in cracking and reforming units for various petroleum fractions, provided with heat-exchange tubes made from such steels and working at temperatures ranging from about 900 to l,050C.

It is also known that austenitic steels of this type have suitable weldability properties. More particularly, the tubes are butt welded by means of a filler metal which is deposited in a groove provided between the two adjacent ends of two tubes to be welded, above a seam forming a weld root and performed in the first place on the internal wall of the mutually confronting ends of the two tubes to be assembled.

It is also known that austenitic steels of this type are not subjected to heat treatments such as for instance annealing, which are not favourable to their mutual welding and which considerably reduce the instantaneous elongation values.

Such austenitic steels entail serious difficulties in use owing to the welds being relatively brittle and resulting in premature ruptures in the welds of the welded assemblies. This applies in particular to the heatexchange tubes used in cracking and reforming units, all the more as the diameter and thickness of the tubes are important.

The brittleness of the welds results in a reduced life of the welded parts, thus leading to high repair expenses and also to high production costs owing to the resulting idle time of the equipment.

The brittleness of the weld regions of heat-resistant steel parts at working temperatures on the order of from 900 to l,050C is avoided by using the method of manufacture according to the present invention. According to this method, heat-resistant high-alloy austenitic steels alloyed with chromium, nickel and/or cobalt, and containing less than 60 percent iron and up to 0.5 percent carbon with small concentrations of additional elements such as, in particular, manganese and silicon, are subjected subsequent to welding to a homogenization heat-treatment at a temperature comprised between about l,lC and l,200C during a period of time of from several minutes to a few hours, the said treatment being followed by controlled cooling. Such a method, quite unexpectedly and contrary to the general teachings of the art, leads to a considerably increased resistance, in particular to creep rupture at the above-mentioned high working temperatures of the welded assemblies of the heat-resistant alloys (steels) having the aforementioned compositions. In particular, the said treatment imparts to the welds a tensile strength at least equal to that of the base metal constituting the welded pieces. This is probably due to the fact that when the method of the invention is applied, there takes place a regeneration of the solidification front of the weld through dissolution of the carbides, re-passing of the carbon into a solution and homogene ous reprecipitation of the carbides in the refusion zone during the cooling process, thus resulting, in particular, in the removal of the physical and chemical discontinuity which otherwise is noticeable, in particular, in the solidification front of the weld.

This appears quite clearly from an examination of the micrographic aspect of the solidification front with a magnification of more than 10,000, i.e., by electronic microscopy means. Indeed, such an examination shows at the solidification front a micrographic structure which is fundamentally different from the one existing before the treatment, from the point of view of the distribution and orientation of the carbides and chemical analysis of the latter.

The invention will be better understood and other objects, characteristics and advantages thereof will appear as the following description proceeds, with reference to the appended drawings given solely by way of example illustrating several forms of embodiment of the invention and wherein:

FIG. 1 is a diagrammatic view illustrating a preliminary stage of the butt welding of two heat-resistant steel parts;

FIG. 2 shows the two steel parts of FIG. 1 assembled by welding and diagrammatically illustrates the general orientation of the crystals depending upon the region considered;

FIGS. 3 and 4 illustrate curves in logarithmic coordinates, showing the tensile strength plotted in ordinates in kg/m as a function of the time, plotted in abscissae in hours, during which the said parts are subjected to a creep action at a given high working temperature, for instance on the order of 950C, for various portions of the said parts prior and subsequent to treatment;

FIG. 5 illustrates a variant ofwelding of two tubes to which the method of the invention may be applied.

Reference is first made to FIG. I which diagrammatically illustrates the abutment of two pieces 10, ll of heat-resistant austenitic steel of the abovementioned grades. The said steel pieces may be constituted for example by plates or by tubes. In the most frequent con templated case of tubes, the internal wall of the tubes is the wall 10a, 11a, whereas the external wall of the tubes is the wall 10b, 11b.

Since the steel pieces considered are relatively thick, the welding is performed by means of a filler metal and by providing a groove 12, the opening of which is directed towards the external face of the tubes between the bevel surfaces 10c, lie of the said tubes. The tubes abut by their heels 10d, lid on the internal side of the tubes.

As diagrammatized in FIG. 2, the welding is performed in two stages.

In a first stage, a root pass is carried out, which forms a weld seam 13 in the region of the heels 10d, 11d. This pass may be performed by simple fusion of the heels 10d, 11d or by means of deposited metal of nearly similar chemical composition or a higher alloy metal.

Once the seam 13 has been formed, the weld is completed by filling the groove 12 by means of successive passes of deposited metal, thus forming the filling seam l4.

The welding may be performed by any known technique, e.g. manual arc-welding, automatic flux or inert gas welding, electron-bombardment welding, friction welding, automatic welding with or without filler wire, etc..

Whatever the technique used, a very high brittleness of the root seam 13 is observed in industrial use as well as laboratory tests, the said brittleness being all the more noticeable as the thickness of the tube is greater. This results in premature ruptures and in a creep breaking strength much lower than that of the filling seam 14.

When the thickness of the parts to be welded increases and exceeds about 12 mm the cooling speed of the re-fused metal of the root seam 13 is very rapid. Moreover, the solidification is oriented (general orientation of the crystals) as diagrammatized in FIG. 2. Otherwise stated, the orientation of the crystallization of the root seam 13 is substantially perpendicular to that of the tubes and Hand of the filling seam l4. 1n the medial portion of the root seam 13 is observed a solidification front 15 which is constituted by a carbide-free austenite border. This chemical discontinuity is a region of premature ruptures in use and in creeprupture test pieces, forming incipient cracks.

This phenomenon considerably increases when the thickness of the parts to be welded exceeds 30 mm.

In order to delay the cooling of the metal of the root seam l3 subsequent to welding, the parts to be welded may be preheated up to a temperature which causes neither precipitation of carbon into solid solution in the matrix, nor the transformation of the primary carbides in M C M C The pre-heating is performed at between 500 and 600C; it enables the welding to be achieved more easily, but'it does not remove the hot brittleness of the solidification front 15 of the root seam 13. The method of the invention enables this brittleness to be removed.

According to the invention and as mentioned in the foregoing, in the case of heat-resistant high-alloy austenitic steels alloyed with chromium, nickel and/or cobalt and containing less than 60 percent iron and up to 0.5 percent carbon with small concentrations of additional elements such as, for example, manganese and silicon, the said steels are subjected, subsequent to welding, to a homogenization heat-treatment at a temperature comprised between about 1,100 and l,200C during a period of time of from several minutes to a few hours, the said treatment being followed by controlled cooling.

The homogenization treatment temperature must be sufficient to dissolve the carbides precipitated during the solidification. The carbon thus re-passed into solid solution diffuses and is distributed uniformly in the matrix of the re-fused metal of the root pass.

The heat treatment thus applied removes the chemical heterogeneity of the solidification front 15.

The temperature of the treatment must be higher than l,050C and preferably comprised between l,l00 and l,200C, depending of course upon the chemical composition of the metal deposited in the weld and that of the base metal.

The duration of the heat treatment at from 1,100 to l,200C is comprised between several munutes and a A few hours, depending upon the thickness of the tubes and the carbon-content of the materials. In practice it is on the order of from one to several hours.

The advantages obtained by applying the homogenization heat-treatment method according to the invention will appear more clearly from the examples which will now be described.

EXAMPLE 1 The base metal is an alloy having the following chemical composition:

C 0.41 Mn 0.97 Si 1.03 Cr 24.9 Ni 21.2; the welding is performed on a 28 mm-thick tube having the above defined composition and by using a filler metal of the same chemical composition. The root pass (root seam 13) is performed by simple fusion of the heels 10d, 11d.

A comparison is made of the rupture times at various temperatures of creep test pieces taken from the weld:

in the region of the filling seam 14,

in the region of refusion or of the root seam 13.

The results are given in the following table.

TABLE 1 Load Rupture time Temp. Test Piece Treatment kg/mm hrs.

Filling seam (14) untreated 1.5 l 620 hrs. Root 950C seam (13) untreated 1.5 790 hrs.

Root seam (13) 1150C Zhrs. 1.5 l 348 hrs. Root 1000C seam (13) untreated 0.9 870 hrs.

Root seam (13) 1150C 2hrs. 0.9 6 084 hrs. Root 1050C seam (13) untreated 0.9 223 hrs.

Root seam (l3) 1150C Zhrs. 0.9 1 805 hrs.

The improvements obtained may be summarized as shown in the following table II (in which 0,; measures the rupture creep stress at the contemplated tempera-. tures and test durations):

In this example, the application of the method of the invention therefore results in a considerable improvement in the creep resistance of the weld root seam l3 and in a slight improvement of the creep resistance of the filling seam 14. The improvement obtained is all the more important as the service time is long.

The curves of FIGS. 3 and 4 illustrate the results given in the above tables I and II. Referring to FIG. 4, it is seen that, whereas for untreated pieces used previously the ruptures in service occurred after about 25,000 hours, i.e., 3 years,.the resistance of the pieces EXAMPLE 2 The base metal is an alloy of the following composition:

C 0.39 Mn 0.92 Si 0.94 Cr 23.2 Ni 34.5 Nb 0.71.

The thickness of the tube and the welding conditions are identical with those of Example 1. The creep breaking tests are performed at 950C. The results of the tests are given in the following table 111:

TABLE III Test Piece Treatment Load Rupture time Filling seam untreated 2 8 507 hrs. Root seam untreated 2 4 466 hrs. Root seam 1150C 2 hrs 2 8 980 hrs. Root seam untreated 1.5 8 652 hrs. Root seam 1150C 2 hrs 1.5 64 940 hrs.

FIG. 5 illustrates the electron bombardment buttwelding of two tubes 16, 17 without filler metal. In this case, before the application of the method of the invention, the solidification front 18 passes through the whole thickness of the tube. It is precisely for this reason that this kind of welding was not applied up to the present, for the welds were highly brittle for the heatresistant alloys under consideration.

On the contrary, when the method of the invention is applied, the creep resistance and the resistance to thermal shocks of the said welds are improved for the reasons mentioned in the foregoing.

It should be noted that in this kind of welding, the electron bombardment gun may be used, subsequent to welding, to perform the homogenization heattreatment according to the invention.

The following example 3 illustrates the results obtained.

EXAMPLE 3 TABLE IV WELD mean a 950C Base metal untreated 1 150C 2hrs.

In this case, the improvement of the weld seam 18 is on the samerorder as that in the two foregoing examples for the root seam 13.

In practice, the homogenization heat-treatment according to the invention is carried out in any suitable manner.

It may be applied either only to the weld region of the pieces or to the whole piece.

As an example of heat treatment according to the invention as applied to a thick tube, for instance 25 mm in thickness, and starting from the ambient tempera ture, the heating of the tube is performed up to the treatment temperature of l,l50C by raising the temperature by 300C per hour. The treatment temperature of 1,150C is then maintained for 2 hours. Thereafter, a cooling down to 800C is performed by lowering the temperature by C per hour. The subsequent cooling is performed in air.

Among other austenitic alloys which may be used, the following ones may be mentioned by way of example:

Steel A.S.T.M. grade HK, i.e., comprising in particular 25 percent chromium and 20 percent nickel.

A.S.T.M. grade l-IT or HU, i.e., comprising 15 percent chromium and 35 percent nickel.

A.S.T.M. grade HX, i.e., comprising 15 percent chromium and 65 percent nickel.

MANAURITE 36X comprising 35 percent nickel, 25 percent chromium and niobium.

Nickel, chromium, cobalt and tungsten alloys marketed under the denominations 22 [-1, super 22 H Blaw- Knox and Supertherm Abex.

Nickel, chromium, aluminum, titanium alloys known under the denominations Incoloy 800 and 802.

Of course, the invention is by no means limited to the forms of embodiment described and illustrated, which have been given by way of example only. In particular, it comprises all the means constituting technical equivalents to the means described as well as their combinations,,should the latter be carried out according to the spirit of the invention.

What is claimed is:

1. Method of manufacturing a welded assembly the welded parts of which withstand working temperatures ranging from about 900C. to 1050C, said parts being of at least one high alloy-austenitic steel alloyed with chromium, nickel and/or cobalt and containing less than 60 percent by weight iron and up to 0.5 percent by weight carbon with small concentrations of additional elements such as manganese and silicon, wherein said parts are submitted to welding, said welding resulting in the formation of at least a weld seam comprising a solidification front corresponding to a physical and chemical discontinuity in said seam, said welded seam being also of said high alloy austenitic steel, and then causing the said solidification front to become chemically homogeneous, causing the carbides of said welded seam to be in the precipitated state at a maximum extent and according to a fine uniform distribution, and causing the resistance to creep rupture and the tensile strength of said weld seam to increase to values at least approximately equal to those of the welded parts all by submitting at least said seam and the adjacent areas of the welded parts to a homogenization heat treatment at a temperature between about C to l200C during a period of time of from several minutes to several hours, thereafter subjecting at least said seam and said 7 adjacent areas to a cooling down to about 800C. with a temperature decrease speed of about 100C/hr and then to an air cooling down to room temperature.

2. Method according to claim 1, wherein prior to welding a pre-heating of the aforesaid parts is performed at a temperature between 500 and 600C.

3. Method according to claim 1, wherein said homogenization heat treatment is performed during a period of time of from one hour to several hours.

4. Method according to claim 1, wherein said weld seam is obtained by depositing the metal thereof onto said parts.

5. Method according to claim 1, wherein said weld seam is obtained by fusion of portions of said parts.

6, Method according to claim 1, wherein said welded parts are two tubes which are butt-welded, said welding comprising the formation of an inner root seam by fusing the inner side of a free end of one of said tubes against the inner side of a free end of the other tube and the subsequent formation of an outer filler scam in an external groove provided between said two ends by depositing the metal of said outer seam in said groove.

7. Method according to claim 1, wherein said welded parts are two tubes which are butt-welded by fusing a free end of one of said tubes against a free end of the other tube, said welding comprising the formation of only one weld seam passing through the whole depth of the tubes.

8. Welded assembly prepared according to the pro cess of claim 1 and comprising two welded parts and at least a weld seam between said parts, both said parts and said seam being of at least one high alloy austenitic steel alloyed with chromium, nickel and/or cobalt and containing less than 60 percent by weight iron and up to 0.5 by weight carbon with small concentrations of additional elements such as manganese and silicon, said seam having no predominant crystallization direction, the solidification front of said seam as Well as the other areas thereof presenting an uniform and fine distribution of carbides, and said solidification front being chemically homogeneous. v 

1. METHOD OF MANUFACTUREING A WELDED ASSEMBLY THE WELDED PARTS OF WHICH WITHSTAND WORKING TEMPERATURES RANGING FROM ABOUT 900*C. YO 105C*C, SAID PARTS BEING OF AT LEAST ONE HIGH ALLOY-AUSTENITIC STEEL ALLOYED WITH CHROMIUM, NICKEL AND/OR COBALT AND CONTAINING LESS THAN 60 PERCENT BY WEIGHT IRON AND UP TO 0.5 PERCENT BY WEIGHT CARBON WITH SMALL CONCENTRATIONS OF ADDITIONAL ELEMENTS SUCH AS MANGANESE AND SILICON, WHEREIN SAID PARTS ARE SUBMITTED TO WELDING, SAID WELDING RESULTING IN THE FORMATION OF AT LEAST A WELD SEAM COMPRISING A SOLIDIFICATION FRONT CORRESPONDING TO A PHYSICAL AND CHEMICAL DISCONTINUITY IN SAID SEAM, SAID WELDED SEAM BEING ALSO OF SAID HIGH ALLOY AUSTENITIC STEEL, AND THEN CAUSING THE SAID SSOLIDIFICATION FRONT TO BECOME CHEMICALLY HOMOGENEOUS, CAUSING THE CARBIDES OF SAID WELDED SEAM TO BE IN THE PRECIPITATED STATE AT A MAXIMUM EXTENT AND ACCORDING TO A FINE UNIFORM DISTRIBUTION, AND CAUSING THE RESISTANCE TO CREEP RUPTURE AND THE TENSILE STRENGTH OF SAID WELD SEAM TO INCREASE TO VALUES AT LEAST APPROXIMATELY EQUAL TO THOSE OF THE WELDED PARTS ALL BY SUBMITTING AT LEAST SAID SEAM AND THE ADJACENT AREA OF THE WELDED PARTS TO A HOMOGENIZATION HEAT TREATMENT AT A TEMPERATURE BETWEEN ABOUT 110*C TO 1200*C DURING A PERIOD OF TIME OF FROM SEVERAL MINUTES TO SEVERAL HOURS, THEREAFTER SUBJECTING AT LEAST SAID SEAM AND SAID ADJACENT AREAS TO A COOLING DOWN TO ABOUT 800*C. WITH A TEMPERATURE DECREASE SPEED OF ABOUT 100*C/HR AND THEN TO AN AIR COOLING DOWN TO ROOM TEMPERATURE.
 2. Method according to claim 1, wherein prior to welding a pre-heating of the aforesaid parts is performed at a temperature between 500* and 600*C.
 3. Method according to claim 1, wherein said homogenization heat treatment is performed during a period of time of from one hour to several hours.
 4. Method according to claim 1, wherein said weld seam is obtained by depositing the metal thereof onto said parts.
 5. Method according to claim 1, wherein said weld seam is obtained by fusion of portions of said parts.
 6. Method according to claim 1, wherein said welded parts are two tubes which are butt-welded, said welding comprising the formation of an inner root seam by fusing the inner side of a free end of one of said tubes against the inner side of a free end of the other tube and the subsequent formation of an outer filler seam in an external groove provided between said two ends by depositing the metal of said outer seam in said groove.
 7. Method according to claim 1, wherein said welded parts are two tubes which are butt-welded by fusing a free end of one of said tubes against a free end of the other tube, said welding comprising the formation of only one weld seam passing through the whole depth of the tubes.
 8. Welded assembly prepared according to the process of claim 1 and comprising two welded parts and at least a weld seam between said parts, both said parts and said seam being of at least one high alloy austenitic steel alloyed with chromium, nickel and/or cobalt and containing less than 60 percent by weight iron and up to 0.5 by weight carbon with small concentrations of additional elements such as manganese and silicon, said seam having no predominant crystallization direction, the solidification front of said seam as well as the other areas thereof presenting an uniform and fine distribution of carbides, and said solidification front being chemically homogeneous. 